CT's have been the principal apparatus for measuring ac currents of nominally 50 or 60 Hz in electrical power systems for many years and are well known. They generally have a torroidal iron core with a secondary winding wound on the core. The primary current to be measured is fed through the core central aperture and the secondary current in the coil, which is an instantaneous analogue of the primary current, is typically supplied to a resistor or other type of electrical load such as a relay coil to give a corresponding analogue voltage for metering, control or protection purposes. The turns ratio of the CT is generally arranged to give a secondary current of either 1 A rms or 5 A rms for the rated primary current. The CT load is generally known as the burden and will be referred to as such herein.
CT's essentially provide a current output which is an analogue of the current to be measured. They are generally not clip-around devices and to replace a CT necessitates disconnection of the conductor to which it is attached. They are also relatively heavy and bulky due to the iron core and copper winding, particularly for large current ratings, and they have the disadvantage that, in the event of the secondary winding being accidentally open circuited whilst primary current flows, an excessive and potentially lethal voltage will be produced across the secondary winding.
Alternative known methods of measuring electric currents are Hall-effect transducers, which also require an iron core, and Rogowski transducers which do not. In contrast to CT's, Rogowski transducers provide a voltage output which is an analogue of the current to be measured (such output also being appropriate for metering, control or protection purposes) and also, in contrast to CT's, these devices are available in a clip-around form.
Rogowski transducers comprise a Rogowski coil and an electronic integrator. They have the advantages of relatively small size and weight, high bandwidth, ability to withstand over-currents without damage, no high voltages following open circuiting the coil or integrator, and no injected impedance into the primary circuit. They are therefore becoming increasingly popular for measuring ac currents in electrical power systems. However conventional Rogowski transducers cannot be retro-fitted to existing current measurement systems in order to directly replace a CT therein unless the existing current monitoring or protection circuitry which the CT supplied is also replaced. This is because, unlike CT's, conventional Rogowski transducers do not provide an analogue current output. Replacing the current monitoring or protection circuitry is, however, costly, inconvenient and time-consuming for the end user.
There is no known means for measuring current which produces an analogue current output and which is also relatively lightweight, safe and straightforward to use.
An invention is set out in the claims.
According to an aspect a voltage to current converter is provided for use with a current measuring device, said current measuring device being operable to provide an output voltage (VI) which is an analogue of a current (I1) to be measured. The converter is arranged to provide an output current (I) which is an analogue of said current (I1) to be measured. Said converter comprises an electronic controller, a switching amplifier, and means for measuring the output current (I) as an analogue voltage (VF) and providing a measure of said analogue voltage (VF) to said electronic controller. It may also comprise a filter connected to the switching amplifier.
According to an aspect a voltage to current converter is provided for use with a current measuring device, said current measuring device being operable to provide an output voltage (VI) which is an analogue of a current (I1) to be measured. The converter is arranged to provide an output current (I) which is an analogue of said current (I1) to be measured. Said converter comprises an electronic controller, a switching amplifier, and a filter connected to the switching amplifier. The filter may be connectable between first and second capacitance values. The capacitance of the filter may be substantially zero. The filter may be comprised as part of a load connected to the converter.
According to an aspect a voltage to current converter is provided for use with a current measuring device, said current measuring device being operable to provide an output voltage (VI) which is an analogue of a current (I1) to be measured. The converter is arranged to provide an output current (I) which is an analogue of said current (I1) to be measured. Said converter comprises the following components: an electronic controller, a switching amplifier, a resistor for measuring the output current (I) as an analogue voltage (VF) and a filter connected to the switching amplifier. Any of the components may be selected for inclusion in the converter based on one or more of its operational characteristics, or based on an operational characteristic of another of the components, so that the converter can operate within specified limits. For example, the components may be selected so that the value of the output current (I) provided by the converter—which is an analogue of said current (I1) to be measured—is accurate to within a predetermined limit under a range of operating conditions.
According to an aspect a current measurement system is provided including a current measuring device operable to provide an output voltage (VI) which is an analogue of a current (I1) to be measured and a voltage to current (V to I) converter, said converter being arranged to provide an output current (I) which is an analogue of said current (I1) to be measured.
According to an aspect a method is provided of manufacturing a voltage to current (V to I) converter and/or a current measurement system substantially as described herein.
According to an aspect a method is provided of designing a voltage to current (V to I) converter and/or a current measurement system substantially as described herein.